Shelf



FIG. 1 is a perspective view showing the new design for a shelf;

FIG. 2 is a front view thereof;

FIG. 3 is a rear view thereof;

FIG. 4 is a top view thereof;

FIG. 5 is a bottom view thereof; and,

FIG. 6 is a right side view thereof, a left side view being a mirror image thereof.

The portions of the shelf in FIG. 3 shown in broken lines are included to show features that form no part of the claimed design. 

CLAIM The ornamental design for a shelf, as shown and described. 